This invention relates to a drive condition changeover apparatus for a transfer unit including a shift rod which is slidably supported in a transfer case and adapted to cooperate with a shift fork to effect changeover between two wheel and four wheel drive conditions.
Generally, a transfer unit receives vehicle engine power through a transmission mechanism and performs such functions as to drive the vehicle in high or low speed drive condition, to interrupt power transmission to drive wheels, and/or to effect changeover between two wheel and four wheel drive conditions.
In a known drive condition changeover apparatus disclosed in the Japanese Utility Model Application Publication No. 16,752/78, when the transfer unit is operated to shift from two wheel and four wheel drive condition, even though the coupling sleeve at first cannot move to the four wheel drive side by collision between teeth which are to be meshed with each other, the coupling sleeve can be displaced after waiting for meshing between the teeth. However, the waiting mechanism cannot operate when the transfer unit is shifted from four wheel and two wheel drive condition. Thus, the shifting operation is not always easy. Further, the waiting mechanism is exposed outside the transfer case, so that a special lubrication system must be provided other than the ordinary one for the transfer case. Also, as the shift rod is connected with the shift fork at one end, and with the waiting mechanism at the other end, the shift rod must be cantilever supported at the middle portion by the transfer case. Thus, a large bending force is applied to the middle portion of the shift rod during the shifting operation, acting as a large frictional resistance. Spring means provided in the waiting mechanism to displace the shift rod must be strong enough to overcome the frictional resistance, and shift operation force of the operation lever must be sufficiently strong to overcome the spring force.